1. Field of the Invention
An automatic transmission typically performs speed shift control according to a previously established shift pattern. The speed shift pattern, in the case of an automatic transmission used in an automotive vehicle, is typically established using engine load and vehicle speeds as parameters. Generally, a plurality of different speed shift patterns are provided. For instance, in the case of two speed shift patterns, one shift pattern is used in an economy operation mode and the other is used in a power operation mode. Speed shifting is made at vehicle speeds which are higher in the power operation mode than in the economy operation mode. These operation modes of the automatic transmission are selectively used by a driver.
2. Description of Related Art
One such automatic transmission has already been reduced to practice and is known from Japanese Unexamined Patent Publication 57-184755. In the automatic transmission described in this publication, when the speed shift pattern for an economy operation mode is selected, the automatic transmission is operated more frequently in higher gears so as to create a travel mode giving priority to fuel economy. On the other hand, when the power operation mode is selected, the automatic transmission is operated more frequently in lower gears so as to create a travel mode giving priority to powerful running.
Some automatic transmissions of this kind automatically select the economy mode and the power mode according to vehicle running conditions. One such transmission is known from, for instance, Japanese Patent Publication 57-8,983. Such an automatic transmission can automatically place the automobile in the fuel economy weighted travel mode or the power weighted travel mode according to vehicle running conditions.
Automatic transmissions are typically equipped with associated frictional elements or mechanisms which include a clutch and a brake which are frictionally locked and unlocked so as to place the automatic transmission into a desired speed gear. Locking and unlocking the frictional elements is managed by line pressure, which is supplied to and relieved from hydraulic actuators. Accordingly, if the line pressure, governing the frictional locking force of the frictional element, is insufficient, a slippage of the frictional elements will occur.
For this reason, the line pressure is typically established by taking into consideration a change of engine torque caused, in a transitional state of the automatic transmission, during an acceleration of the automobile. Because the engine changes torque greatly during acceleration, and a frictional force required to lock the frictional element is large, the line pressure is designed so as to generate a frictional force which is sufficiently high to lock the frictional element during the transitional state. However, since the change in engine torque is small during steady travel, a line pressure determined in this way may be unnecessarily high for the frictional elements.